1. Field of the Invention
The present invention relates to a zero-phase sequence current detector for detecting occurrence of zero-phase sequence current due to a ground fault.
2. Description of Prior Art
A conventional zero-phase sequence current detector shown, for example, in "KEIKI YOH HENATSUKI" (Mihoji IKEDA: DENKI SHOIN, Japan P.79, 1962) is described referring to FIG. 6.
In FIG. 6, a circular core 2 is provided for enclosing three power lines of respective phases 1a, 1b and 1c. A secondary winding 3 is wound on the circular core 2. A secondary burden 4 is connected to the secondary winding 3.
In stationary case, currents flowing on the power lines of the three phases 1a, 1b and 1c do not contain any components due to an accident, and the respective currents are AC current consisting of positive phase components and opposite phase component respectively having the same amplitude. Magnetic field in the circular core 2 is a composition of magnetic fields induced by the currents flowing on respective phases and resultantly becomes null. As a result, any current does not flow on the secondary winding 3, and the second burden 4 is not driven.
On the contrary, if a ground fault occurs on any one of the power lines of the three phases 1a, 1b and 1c, a sum current value composed bvy the currents flowing on respective phases does not become null. As a result, a zero-phase sequence current flows. When the zero-phase sequence current flows, a magnetic field is induced in the circular core 2 and a secondary current responding to the zero-phase sequence current flows on the secondary winding 3. The secondary burden 4 is driven by the secondary current flowing on the secondary winding 3, and thereby the power lines of the three phases 1a, 1b and 1c are tripped from an electric power source. As a result, magnification of the ground fault is prevented.
Generally, it is desirable that a zero-phase sequence current detector is preferably to be disposed not only in substations but also on overhead lines in the open air for quickly finding out the location where the ground fault occurs and for restricting the power failure area as small as possible.
The conventional zero-phase sequence current detector, however, is constituted to use a circular core 2 which encloses the power lines of the three phases 1a, 1b and 1c together. Accordingly, there are problems that the conventional zero-phase sequence current detector are not suitable to be disposed on the overhead lines and that an accident may be large at a thunder bolt.
Furthermore, when the zero-phase sequence current detector is miniaturized and disposed on the respective phases, an apparent zero-phase sequence current is generated by differences of the temperature characteristics of the respective current detecting devices disposed on the respective phases, and thereby there is a problem that the secondary burden 4 makes maloperation.